The present invention relates to an image forming apparatus which includes a belt-shaped image bearing member unit that lays an image bearing member configured by an endless belt around a drive roller and a winding member to drive and rotate the image bearing member, and which forms a toner image on the image bearing member, and thereafter transfers the toner image onto a recording medium to form an image.
In the image forming apparatus, a charging device for charging a photoconductor uniformly in a rotary direction, an exposing device for forming an eletrosatc latent image on the photoconductor, a developing device for developing the electrostatic latent image, a transferring device which transfers a toner image on the photoconductor onto a transfer medium, and a cleaner which cleans a face of the photoconductor after transfer, are arranged around the periphery of the photoconductor serving to an image bearing member. The electrostatic latent image formed on the photoconductor by the exposing device is developed by the developing device thereby to form a toner image. The formed toner image is transferred onto the transfer medium thereby to form an image. After the toner image is transferred, the residual toner on the photoconductors is removed from the face of the photoconductor by the cleaner.
In case of a rotary development type full-color image forming apparatus which forms a full-color image, a rotary development device provided with respective developing units of yellow Y, magenta M, cyan C, and black K forms a toner image of each color on the photoconductor, the toner images of the respective colors are in order transferred onto an intermediate transfer medium for multi layer transfer. Therefore, a development cartridge of each toner is detachably mounted on a development rotary unit; and each time an electostatic latent image for each color is formed on the photoconductor, the development rotary unit is driven by a development color switching operation, and the corresponding development cartridge is rotated and moved to the development position thereby to perform the developing operation for example, refer to JP-A-2002-287461 and JP-A-2003-5511).
FIG. 5 is a diagram showing outlines of the constitution of the full-color image forming apparatus. In the full-color image forming apparatus, as shown in FIG. 5, respective developing units 22Y, 22M, 22C, and 22K of yellow Y, magenta M, cyan C, and black K are detachably mounted on a rotary development device 21. The belt-shaped image bearing member unit includes a drive roller 7, a driven roller 8, a tension roller 16, and an endless belt. Further, the image bearing member unit includes an intermediate transfer belt 3 laid around the rollers 7, 8 and 16, and rotation-transferred in a direction of an arrow, on which a toner image on a photoconductor 20 is transferred; a first bias transfer roller 15 which is arranged on the back face of the intermediate transfer belt 3, opposed to the photoconductor 20, and transfers firstly the toner image on the photoconductor 20 onto the intermediate transfer belt 3; a cleaner blade 14 which removes residual toner on the intermediate transfer belt 3; and a second bias transfer roller 17 which is arranged opposed to the drive roller 7, and transfers secondarily a full-color toner image of four colors formed on the intermediate transfer belt 3 onto a recording medium (for example, a paper) 18.
In the image forming operation, selective exposure according to image data of a first color, for example, yellow Y is performed on the face of the photoconductor 20, and an electrostatic latent image of yellow Y is formed. At this time, the rotary development device 21 rotates and moves so that a development roller of the development cartridge 22Y of yellow Y comes into contact with the photoconductor 20, a toner image of the electrostatic latent image of yellow Y is formed on the photoconductor 20, and sequentially the toner image is transferred onto the intermediate transfer belt 3 by the first bias transfer roller 15 to which a first bias of an opposite polarity to the toner charged polarity has been applied.
For this time, the cleaner blade 14 and the second bias transfer roller 17 are separated from the intermediate transfer belt 3. This series of processing is repeatedly executed correspondingly to each image forming signal of a second color, third color, and a fourth color, whereby toner images of yellow Y, magenta M, cyan C, and black K according to contents of each image forming signal are in order transferred from the photoconductor 20 onto the intermediate transfer belt 3 in a multi layered manner thereby to form a full-color image of four colors.
Then, at a timing when the image formed by multilayer-transferring the toner images of the respective colors reaches the second bias transfer roller 17, the recording medium 18 in a sheet supply tray is transported from a pick up roller through a registration roller and a sheet material transporting path to the second bias transfer roller 17, the second bias transfer roller 17 is pressed on the intermediate transfer belt 3 and receives a second bias, and the toner image on the intermediate transfer belt 3 is transferred onto the recording medium 18 by the second bias transfer roller 17. The transfer medium on Which the toner has thus been transferred is transported to a fixing unit by a paper transporting unit, and the toner image on the recording medium 18 is heated and pressed by the fixing unit so that the toner image is fixed on the recording medium. The residual toner on the intermediate transfer belt 3 is scratched and removed by the cleaner blade 14 that has come into contact with the intermediate transfer belt 3.
FIG. 6 is a diagram for explaining drive reaction force and a sink phenomenon which act on a drive gear of the belt-shaped image bearing member unit, and FIG. 7 is a diagram for explaining an attachment structure of the belt-shaped image bearing member unit.
In the above related image forming apparatus, when the deaner blade 14 comes into contact with the intermediate transfer belt 3, a large load is rapidly applied onto the intermediate transfer belt 3, so that a drive transmitting gear 6 applies drive reaction force P onto the drive gear 5 as shown in FIG. 6. The belt-shaped image bearing member unit as shown in FIG. 7, is fixed, at its going part having four filing legs 2-1 to 2-4 (in FIG. 7, only the leg 2-1 is shown) which protrude from a unit frame 2, to a body frame 9 by locking screws 13. This fixing part is away from a supporting part which supports a bearing 4 for supporting a rotary shaft of the drive roller 7 and a drive gear on the unit frame 2.
Therefore, as shown in FIG. 6, when the drive transmitting gear 6 applies the drive reaction force P onto the drive gear 5, deformation (strain) is produced in the unit frame 2, and the drive gear 5 of the drive roller 7 for driving the intermediate transfer belt 3 sinks downward. Therefore, the drive power is not transmitted from the drive transmitting gear 6 on the image forming apparatus body side to the drive gear 5 on the belt-shaped image bearing member unit side, so that a problem that peripheral speed of the intermediate transfer belt becomes lower than that of the photoconductor 20 is caused. Since variation of the belt peripheral speed is caused lastly, a color registration error of the color transferred on the belt in a contact state of the cleaner blade 14 is caused with respect with a color transferred on the belt in a non-contact (separation) state of the cleaner blade 14. The registration error is produced in a direction where the belt lags behind (in a paper leading end direction).
For the purpose of eliminating such the disadvantage, in order to prevent a position shift of the drive roller in the drive reaction force direction due to the load of the separation and contact member, a reduction of the energizing force of the separation and contact member composed of the cleaner blade 14 or reinforcement of the unit frame 2 (formation using a hard material) has been performed. However, when the energizing force of the cleaner blade 14 is lowered to reduce the load, cleaning property on the intermediate transfer belt 3 lowers. Further, when the rigidity of the unit frame 2 increases to suppress the deformation of the unit frame 2, the weight of the unit frame 2 increases, which causes the increase of cost.